User blog:Samuli.seppanen/New record shots
Yesterday I finally had shooting session with the almost completely reconstructed cheiroballistra. Here's some background information * Springs were tuned to ~350hz (up from ~320hz) * The distance between torsion springs was correct ** The (nylon) bowstring was significantly shorter than previously * Anchor points for the handle were at same places as in the previous case * Cones were now correctly 11 dactyls long ** Draw length was reduced by ~3 dactyls (~6cm) * Stomach pressure alone was used for cocking * A new slightly longer quadrangular handle was used ** Draw length further reduced by ~1 dactyl (~2cm) In previous cheiroballistra incarnation cones were incorrectly 14 dactyls long. This was only possible because the little arch and ladder were too long (see above). Back then I had ended up "amending" the cone length in my hunt for longer draw (see previous blog posts). Overall, the cheiroballistra now works perfectly. In fact, there were zero issues with the operation. That said, when the power was ramped up to the full, the session ended up in rather uneventful dry-fire during the cocking phase: the hooks in the arms unwound and the cones popped out of the torsion springs. I'm rather surprised that the bowstring did not break and there was almost zero damage on the cones themselves. The reason for the dry-fire seemed to be bowstring slipping from under the claw - a problem which can be fixed fairly easily, e.g. by * Sinking the fingers of the claw into the wood * Making sure the fit between claw's back and trigger's top surface is very tight Anyways, the results of session were pretty good. I shot with the same bolt as the last time which weighted 49.70 grams, slowly ramping up power. Here are the results: It should be note that due to the shorter cones (-3 dactyls) and longer handle (-1 dactyl) the draw length was significantly shorter (-4 dactyls) than previously, yet the results were better than the last time. I think this was only possible because of higher linear pretension (350hz). I think that I can reach 130 joules with stomach pressure, but that's about the limit. I did try to use a pulling handle, but encountered a new problem: as the handle has to be pulled upwards to make it pass over the locking fork (more details later), the slider actually got wedged into the dovetail in the case before full draw. I actually had to tap the slider down in to release it. I did not have this problem previously, which was probably due to less exact fit between the slider and the case. The solution I thought of is to make a locking fork where the fork points downwards instead of upwards. Then I will pull the handle backwards and down, and lift it up after it has passed the fork. That way the slider will not get wedged into the dovetail due to upwards pull. Plus I don't have to pull the handle up at the most inconvenient moment, i.e. at full draw. I'm now at the point where I've learned as much from nylon springs as I can. The next steps: * Order horsehair and start making it into rope for the springs * Bend the hooks in the arms back to shape * Make sure that bowstring can't slip from under the claw * Test lighter bolts ** Assuming the drop in efficiency is reasonable, 30 gram bolt should fly ~85m/s (108J) and a 20 gram bolt ~97m/s (94J) * Plot force-draw curve * Test full draw with 80 degree washer rotation ** Should yield ~120 joules, if doable at all * Make a reverse locking fork to see if/how much a pulling handle can increase energy output ** Might just allow getting 150 joules out of the weapon with some practice and aggressive attitude * Make a miniature onager (as discussed before) for testing various kinds of spring cord materials More details regarding the new parts will follow later. Category:Blog posts Category:Backup Category:Statistics Category:Cocking devices Category:Case and slider Category:Lock